High performance fighter aircraft such as the F-18 and F-22 (available from the Boeing Company of Chicago, Ill.) often experience high frequency aerodynamically induced vibrations of their wings, stabilators, and vertical tails. These vibrations are cause by buffeting aerodynamic forces and are transmitted into, and through, the aircraft structure. If uncompensated for, the associated stresses may lead to premature cracking of the structure. The resulting repairs of these cracks are both expensive and time consuming. In the alternative, aerodynamic modifications to reduce the causative turbulence impose performance constraints on the aircraft while structural modifications to reduce the resulting fatigue stress incur weight and cost penalties.
Additionally, the buffeting of the vehicle transmits noise into, and causes noise within, the aircraft structure. In turn, the structure transmits the noise to the cockpit wherein noise control measures, with additional penalties must be undertaken. Nor are these problems isolated to high performance aircraft. Automobiles, missiles, and launch and reentry vehicles (for example) also receive buffeting from aerodynamic forces.
Various attempts have been made to use the existing flight control actuators to compensate for these aero-vibrations. However, the flight control system typically commands the actuator at rates of about 30 cycles per second or less. Since the vibrations occur at frequencies significantly higher than the commands, such attempts have failed.
Moreover, attempts to modify the flight control system to react quickly enough to respond to the vibration are impractical for a variety of reasons. For instance, such modifications require an order of magnitude increase in the flight computer speed. Thus, these solutions necessitate an upgrade of the computer. Additionally, modifying the flight control laws to accommodate the additional functionality necessitate the recertification of the flight control system. These recertifications are expensive, time consuming, and (as such) highly undesirable.
Furthermore, as composite materials replace aluminum, and other conventional, structural members (e.g. on the Boeing 7E7 aircraft) vibration control may assume an increasing importance in the design, operation, and maintenance of aircraft. Thus a need exists to reduce or eliminate aerodynamically induced vibrations.